We propose to screen wild mice for Ia antigens which are serologically related to I-Ab, I-Ad, I-Ak, or I-Ap, and to selectively breed H-2 homozygous mouse lines with wild-derived H-2 haplotypes containing minor variants of these alleles. Preliminary studies have demonstrated that about 1 of every 5 wild mice will carry an allele whose products are serologically similar, but not identical, to one of these inbred line I-A products. A collection of such wild-derived minor variant I-A alleles will be produced by selectively breeding wild mice as semicongenic H-2 homozygous mouse lines. The structural, functional, and serologic properties of the I-A and I-E products of these lines will be thoroughly analyzed. They will be tested serologically with an extensive panel of anti-Ia alloantisera and monoclonal anti-Ia reagents. The alpha and beta subunits of the A and E molecules of serologically-related inbred and wild mice will be compared by tryptic peptide mapping procedures. The minor structural differences which distinguish the wild-derived variant and inbred line I-A products will be localized to specific fragments of the alpha or beta subunits of the A molecule. The functional differences exhibited by these minor I-A variants will be tested with the primed lymphocyte typing assay, and the lymphocyte-stimulating (LS) determinants which are unique to each minor variant allele will be described. The data will be correlated and the following questions will be answered: 1. Do all the variants of a single allele have a common evolutionary origin? 2. Are all minor structural differences in the I-A or I-E molecules equally effective at stimulating T cell proliferative responses? 3. Are structural variations occurring in certain regions or fragments of the molecules more effectively recognized by antibodies, T cell, or both?